1. Field of the Invention
The present invention relates to a filter circuit including an external control terminal.
2. Description of the Related Art
A filter circuit including an external control terminal is mounted, for example, between an antenna and an IC for transmission and reception. In such a filter circuit, a signal path for transmission and reception is switched by the external control terminal. As an existing filter circuit, for example, there is a filter circuit shown in FIG. 12.
FIG. 12 is a circuit diagram showing an existing filter circuit 1P (see CC1120, p23, [online], Texas instruments, [searched on Sep. 18, 2012], the Internet <URL: http://www.ti.com/lit/ds/symlink/cc1120.pdf>). The filter circuit 1P includes an antenna-side circuit 11P, a transmission-side circuit 12P, and a reception-side circuit 13P.
The antenna-side circuit 11P is connected to an antenna-side terminal T1. The transmission-side circuit 12P is connected to a transmission-side terminal T2 and the antenna-side circuit 11P. The reception-side circuit 13P is connected to reception-side balanced terminals T4 and T5 and the antenna-side circuit 11P. A connection point 16 between the antenna-side circuit 11P and the reception-side circuit 13P is connected to an external control terminal T11.
The antenna-side terminal T1 is connected to an antenna AT. The transmission-side terminal T2 is connected to an external circuit TX. The reception-side balanced terminals T4 and T5 are connected to an external circuit RX. The external control terminal T11 is connected to an external circuit CTL.
The antenna-side circuit 11P includes a low pass filter. The transmission-side circuit 12P includes an LC parallel resonant circuit. The reception-side circuit 13P includes a balanced-to-unbalanced conversion circuit. It should be noted that a frequency band used for transmission and a frequency band used for reception overlap each other.
At the time of transmission, the impedance of the external circuit CTL is set to be sufficiently low. Thus, the external control terminal T11 is grounded, and the potential of the connection point 16 becomes a ground potential. As a result, the reception-side circuit 13P and the external circuit RX do not influence the characteristics of the filter circuit 1P. Thus, a transmission signal inputted to the transmission-side terminal T2 passes through a transmission filter circuit composed of the antenna-side circuit 11P and the transmission-side circuit 12P, and is outputted from the antenna-side terminal T1 to the antenna AT.
At the time of reception, the impedances of the external circuit CTL and the external circuit TX are set to be sufficiently high. Thus, the filter circuit 1P becomes equivalent to a reception filter circuit composed of the antenna-side circuit 11P and the reception-side circuit 13P. Thus, a reception signal inputted to the antenna-side terminal T1 passes through the low pass filter of the antenna-side circuit 11P, is converted to a balanced signal by the balanced-to-unbalanced conversion circuit, and is outputted from the reception-side balanced terminals T4 and T5.
In other words, it is possible to switch between transmission and reception by controlling the impedance characteristics of the external circuit CTL. The filter circuit 1P becomes equivalent to a transmission filter circuit at the time of transmission, and becomes equivalent to a reception filter circuit at the time of reception.
The external circuit CTL shown in FIG. 12 is, for example, an IC (Integrated Circuit). In general, the circuit elements constituting the filter circuit 1P and this IC are mounted on different motherboards. Thus, a line connecting the external control terminal T11 to the IC is long to some extent and thus has a parasitic inductance. As a result, there is a concern that the external control terminal T11 is not fully grounded.
In this case, the connection point 16 is short-circuited to the external control terminal T11, and thus the transmission filter circuit composed of the antenna-side circuit 11P and the transmission-side circuit 12P is influenced by the parasitic inductance of the line. The parasitic inductance of the line is changed in response to the line length, and thus the characteristics of the transmission filter circuit are influenced by the line length of the line and become unstable.